It is indeed a difficult decision to make. I myself have several characters by now, one of each class I like, because it is so difficult to decide for one as main.
Some classes I like the most for PvE and why:
Monk = you are accepted in pretty much any PuG group, but people usually will expect you to heal well. There's some pressure on not letting others die and in harder areas you'll need more concentration than playing other classes. Healing Hench/Hero teams when you are solo can be boring too, so expect to run some PvE damage skill gimmick when playing alone (Assassins Promise, Assassin Support, Finish Him, etc).
Mesmer = good shutdown and damage specially in Hard Mode where it's not so easy to just steamroll everything. Fast casting might not be as strong as Soul Reaping for example, but it is a lot of fun to cast spells fast. Disadvantages are weaker Energy Management compared to Necro and Ele, but Mesmers look great and are capable of a variety of builds.
Paragon = SY! and TNTF! pretty sure guarantee your party will not wipe unless you are totally shut down. Ideal for Legendary Survivor or people who don't like dying at all. Disadvantage is that not all the time all this amount of defense is needed, and Paras lack good builds.
Warrior = good damage, utility and staying power. All weapon masteries have viable builds which guarantees the class is a lot of fun to play. You can deal high DPS, KD mobs, bring SY!, etc. Plus you look damn tough (if male, lol). Strong class.
Assassin = good damage and farming potential, fun class to play with many effective builds available. Nowadays the obvious answer as a title hunting class would be Assassin thanks to Shadow Form, so you can farm gold and buy the buyable titles faster... However, nothing stops you from making a character to farm (sin), and then pass all the cash to your true main character.
The other five classes I don't like so much in PvE.
Ele = Good to farm, good damage in NM, good support in HM. However the damage is not good as expected in HM. I quit mine because I wanted a damage dealer which she wasn't at all... She had 15+ maxed titles. That being said, Eles are a lot of fun if you like variety and support while dealing some damage.
Necro = I hear these are good but I just can't stand their ugly looks, the slow casts, the builds (MM bores me, curses too...). I only really liked Discord but it got old fast. Necro deleted.
Ranger = Despite its awesome disruption power and utility, I ended up quitting my ranger due to low damage in PvE compared to other physical classes. I suck at interrupting, too. lol.
Dervish = Things started out great but I ended up quitting my Dervish because he felt slow and fragile compared to Asn and War. Perhaps I'm just bad at being a Dervish.
Ritualist = The class concept never attracted me much, it felt slow. I don't like Spirits, I prefer to heal as a Monk, I prefer to cast offensive magic as a Mesmer. Just not my style really but with the recent buff the Rts seem faster paced now.
This is my view today about the professions... of course it is all down to personal style, for me the things that make or break a class are:
1st. Pace of speed the class plays; I like faster and more active moving/casting/attacking. Monk requires a lot of speed and most of Monk spells are fast, Mesmer also feels fast, so does Assassin, Warrior and Paragon high attack rates and good IAS. Classes like Necro where I (slowly) cast a curse and wait things to die really kill my experience.
2nd. Looks and armor.
3rd. Grouping potential.
I wrote a LOT, lmao. I hope it helps and wish you good luck choosing your main.
